Although many filamentary materials are known for their utility as thermal insulators, there is still need for higher efficiency systems and lower cost fabrication procedures. Today's shuttle tiles, for example, are produced in a costly process from melt-spun filaments of SiO.sub.2. To produce the desired tile with controlled volume fraction, the SiO.sub.2 filaments are chopped up, randomized, placed in a mold and partially sintered under pressure at elevated temperatures. The sintering process is designed to achieve a specific volume fraction of interwoven filaments that are firmly bonded at points of contact in a random array. Such an open structure has high thermal impedance coupled with modest structural strength, which makes it ideal for shuttle tiles and other insulating purposes where thermal insulation for a transitory period is required.
Our novel synthesis method of a comparable insulating material would involve growing an interwoven network of carbon substrate filaments within a shaped mold, formed in the shape of a shuttle tile, coating the filaments with a Si containing deposit by chemical vapor deposition (CVD), followed by oxidation to eliminate the carbon filament core. The volume fraction as well as the bridging of the filaments is easily controlled by this method as discussed in copending application Ser. No. 113,986, filed Oct. 29, 1987. An added advantage of this process, in addition to its low-cost, is the ability to produce ultra-fine, less than 1 micron in diameter hollow filaments, with controlled diameter and wall thickness. For the same volume fraction, such hollow structures should be even more effective in impeding heat transfer, while sacrificing little in structural strength.
The novel material of this invention may also find utility as a potential replacement for environmentally hazardous asbestos in insulating materials employed in the construction industry. The micro-tubular material of this invention mimics the structure of asbestos, i.e., it is in the form of thin-walled hollow tubular filaments.
Catalyst supports are generally ceramic materials such as silicon dioxide or aluminum oxide prepared with high surface areas in the form of pellets. The novel material of this invention, if grown in the shape of a brick as for the shuttle tile, can also be employed to construct a very high and controllable surface-area catalyst support bed of predetermined dimensions by stacking the bricks. Such a porous body, now of macroscopic dimensions, can now be loaded with catalyst particles by known methods, for example, liquid infiltration.
Exactly the same concept can also be exploited to fabricate a fixed bed for simple filtration purposes, for example to remove dust particles from the air, or for more sophisticated chemical separations when the surfaces of the filamentary networks have been appropriately pretreated, e.g., to give a chemically absorptive surface.
By the method of the present invention it is also possible to entirely fabricate ultra-fine composites that are reinforced with a three-dimensional, tubular network. This has very broad implications for the design of advanced high specific strength composite structures. In the design of such composites for structural applications, thin-walled tubes are preferred reinforcing, or load-bearing elements. This is because tubes make better use of the intrinsic structural strength of materials than rods of the same dimensions. In engineering practice, tubular elements are frequently linked together to form a three-dimensional structure of great strength and flexibility, e.g., as in a geodesic dome. In nature, similar engineering principles are exploited, but with the added complication that the hollow, or cellular structures are themselves composites of intricate design. Networks of cellulose fibers provide much of the reinforcement in natural composites, e.g., trees, grasses, bamboos. Although many attempts have been made to mimic such natural composite designs, so far these efforts have met with little success, primarily because of the difficulty of making hollow filaments with appropriately small dimensions.
By the method of the present invention, almost any desired filament-filler matrix combination can be produced by utilizing chemical vapor deposition (CVD) to modify the surface properties of the filamentary micro-tubular material. Infiltration of filler matrix materials can be achieved by adaptation of existing materials technologies.
Yet another application for these porous structures is as substrates for the recently discovered thin superconducting oxide layers. These could be applied by sol-gel techniques, i.e., dipping and draining, or by more advanced techniques such as by CVD from multiple sources. After deposition on the surface of the filaments either an annealing treatment below the melting point, or a brief melting operation may be necessary to coarsen the grain structure of the superconducting phase. It is known that an alumina substrate is ideal for this purpose, in that good wetting between the alumina and the high Tc superconducting oxides occurs, without significant chemical degradation if the exposure time is brief. It is understood that all processing steps be carried out in an oxygen-rich environment in order to maintain the desired stoichiometry of such superconducting phases. After coating the filaments with the desired thickness of the superconducting phase, the structure may be infiltrated with a polymer to achieve the desired flexibility and strength for structural applications such as filament windings for energy storage and transmission.
It is clear from these examples that there are many uses for such novel materials comprised of interwoven networks of ceramic hollow filaments, whether coated or not. Other applications not listed can be envisioned which are obvious to those skilled in the art.